Asphalt can be obtained, in particular, from crude petroleum, by distillation thereof and/or by deasphalting the heavy fraction obtained by distillation or distillations. Depending on the origin of the crude petroleum, asphalt consists of varying proportions of paraffinic oils (aliphatic or naphthenic), aromatic oils, resins and asphaltenes (on this subject, attention is directed to European Patent [EP] 246 956 and its U.S. equivalent U.S. Pat. No. 4,810,367 both of which are owned by Applicants' assignee and are herein incorporated by reference).
Asphalt is a viscoelastic material: it has a tendency to harden and become brittle at low temperatures; and, it softens and can flow at elevated temperatures. Its temperature sensitivity varies depending on its origin and production method.
It is well known to modify asphalt by addition of a polymer. The polymer is added for the purpose of reducing the heat sensitivity of asphalt by extending its plasticity range (it increases softening temperature and/or reduces cold brittleness temperature). The addition of a polymer to asphalt poses compatibility problems leading to instability of the asphalt/polymer blend during storage, for example before the blend is applied to a road.
It is also known that the addition of a polymer to an asphalt does not increase appreciably the aging resistance of the asphalt as measured by the well-known rolling thin film oven test (RTFOT) (see AFNOR* method T66-032).
An asphalt is thought to be aging when its penetrability (determined by AFNOR method NFT66-004) does not decrease much as a result of the RTFOT test and when its softening temperature (determined by AFNOR method NFT66-008) does not increase much as a result of the same test.
It is also already known to modify asphalts chemically by grafting onto them a monoanhydride such as phthalic anhydride or maleic anhydride, as described in the journal "Materiales de construccion" V37 H205-1987, pages 49 to 56, and thus allegedly markedly reduce their heat sensitivity. However, Applicants or Applicants' assignee have found that the anhydrides described in this publication do not give satisfactory results.
British Patent [GB]-A-2 015 536 is directed to the grafting of a dihalopolybutadiene onto asphalt in the presence of zinc oxide as catalyst. The modified asphalt thus obtained can be blended with an elastomer, for example a polychloroprene or polynorbornene, or with a sequenced polymer, for example styrene-butadiene.
In contrast, Applicants have developed improved asphalts which at the same time exhibit reduced low-temperature sensitivity, good rheological properties at low and high temperature, good aging resistance and improved cohesiveness, without adding a polymer which would have the aforesaid drawbacks, and without adding a compound, such as a monofunctional compound, which would obtain unsatisfactory results.